15．
Scientists Detect Gravitational Waves
What is gravitational waves （引力波）？Scientists have for the first time observed ripples in the fabric of space time （時(shí)空漣漪） called gravitational waves，arriving at the earth from a severely destructive event in the distant universe．It confirms a major prediction of Albert Einstein's 1915general theory of relativity and opens a unique new window onto the universe，according to a group of scientists at a press conference in Washington on Thursday．
"This is truly scientific moonshot．We did it．We landed on the moon，"declared David Reitz，executive director of the LIGO Laboratory at Caltech，at the conference in the National Press Club．
According to the National Science Foundation （NSF） experts，gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot be obtained from elsewhere．Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second （千分之一秒） of the combination of two black holes to produce a single，much bigger turning black hole．This fierce shock of two black holes had been predicted but never observed by NSF．
The gravitational waves were detected on Sept 14，2015at 5：51am EDT by both of the twin Laser Interferometer Gravitational-wave Observatory （LIGO） detectors，located in Livingston，Louisiana，and Hanford，Washington．
Based on the observed signals，LIGO scientists estimate that the black holes for this event were about 29and 36times the weight of the sun，and the event took place 1.3billion years ago．About three times the weight of the sun was changed into gravitational waves in a fraction of a second--with a peak power output about 50times that of the whole visible universe．By looking at the time of arrival of the signals--the detector in Livingston recorded the event 7milliseconds （毫秒） before the detector in Hanford--scientists can say that the source was located in the Southern Hemisphere，according to a press release from NSF，which funded the research．
This new LIGO discovery is the first observation of gravitational waves themselves，made by measuring the tiny disturbances the waves make to space and time as they pass through the earth．"Our observation of gravitational waves accomplishes an ambitious goal set out over five decades ago to directly detect this puzzling phenomenon and better understand the universe，and，properly，fulfills Einstein's prediction on the 100th anniversary of his general theory of relativity，"Reitze said．
74．By saying"This is truly scientific moonshot．We did it．We landed on the moon，"what does David Reitz mean？D
A．We humans truly landed on the moon this time．
B．The theory of relativity was not proved until today．
C．Gravitational waves arrived at the earth in the end．
D．Scientists successfully observed gravitational waves．
75．What do NSF experts talk about in the third paragraph？C
A．Gravitational waves carry information about the origins of nature．
B．The nature of gravity cannot be obtained from gravitational waves．
C．The combination of two black holes can produce a single，much bigger turning black hole．
D．Gravitational waves only appear at the final fraction of a second of the shock of two black holes．
76．According to the observed signals，LIGO scientists find out thatC．
A．the two black holes which brought about this event were much bigger than the sun
B．a(chǎn)bout three times the weight of the sun became gravitational waves in this event
C．the event produced by the observed signals took place 1.3billion years ago
D．the peak power output was about 50times that of the whole universe
77．From this passage，a conclusion can be drawn thatA．
A．gravitational waves can make disturbances to space and time
B．Einstein predicted the observed gravitational waves in the universe
C．gravitational waves is not a puzzling phenomenon to the world any more
D．this new LIGO discovery was made to test the general theory of relativity．
